Use It AND Lose It?

One of my spiritual teachers, the late Eknath Easwaran, used to champion the cultivation of willpower for pursuing the spiritual path. There's no doubt that willpower is something I desperately need to cultivate if I'm to have any hope of achieving even a modest fraction of my goals, spiritual or otherwise.

I've just read an article that reinforces my conviction. It reports a recent psychological study showing that self-discipline was twice as good at predicting academic success than was IQ. Students who were able to reject instant gratification in favor of long-term academic goals were far more likely to do well academically than were the other students, no matter how "smart" they were.

However, the author of this article proposes one caveat. Though self-discipline may be kind of like a muscle that can be strengthened through various forms of exercise, it may also have limits that, if overtaxed, can lead to overall fatigue and collapse. That is, one should be selective about what one wants to truly excel at and channel most of one's willpower into that area while slacking off in most others.

I'm sure Easwaran would have disagreed with this. What do you think about it?